Bump-on-Trace (BOT) structures were used in flip chip packages, wherein metal bumps are bonded onto narrow metal traces in package substrates directly, rather than bonded onto metal pads that have greater widths than the respective connecting metal traces. The BOT structures require smaller chip areas, and the manufacturing cost of the BOT structures is low. The conventional BOT structures may achieve the same reliability as the conventional bond structures based on metal pads.
The BOT structures used to include a solder mask layer that is formed on the metal traces. The solder mask layer covers portions of the metal traces, and leaves some openings, through which the metal traces are exposed. During the bonding process, solder bumps extend into the openings, and are bonded to the exposed portions of the metal traces. The solder mask layer provides mechanical support for the BOT structures, and the metal traces are unlikely to peel off from the underlying structures.
With the evolving of bump structures, the solder mask layer may be omitted or removed from some areas of the package substrates. For example, in the areas where the metal traces have fine pitches, the solder mask may be removed, while in the areas wherein the metal traces have great pitches, the solder mask may be left un-removed. Accordingly, the metal traces in at least some of the areas of the package substrates are exposed when the bond is formed. However, the metal bumps are typically wider than the metal traces, and hence the solder bonding the metal bumps to the metal traces may shift. Several problems may be resulted due to the metal bump shift. For example, solder bumps may crack, or may bridge to neighboring metal traces, particularly when the solder bumps fail to contact the sidewalls of the metal traces. In addition, the solder bump that is bonded to a metal trace may have solder extension along metal trace surface, and hence the current areas may be reduced. This in turn may cause current entrance areas to be reduced to equal to or smaller than the widths of the metal traces. As a result, the current density in the solder bumps is increased, and high electro-migration is resulted.